On Thu, 14 Sept 2023 at 19:12, Jonathan Wakely <cxx_at_[hidden]> wrote:

>
>
> On Thu, 14 Sept 2023 at 16:06, Grzegorz Sikorski via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> Thanks all for your answers. I obviously misread the description in
>> https://en.cppreference.com/w/cpp/ranges/filter_view (in other places
>> the overload for a projection is explicitly shown). Yes, this helps a lot
>> and solves one problem. However the original issue I was trying to fix
>> recently was this: https://godbolt.org/z/TW8P5xqxz. I use range-v3 as a
>> polyfill for the missing std::views::chunk_by, maybe this is the problem.
>>
>
> Firstly, std::views::chunk_by is not missing, it's part of C++23 so
> available if you use -std=c++23
>
> Thanks, I missed this!

>
> Secondly, your attempts to use it are incorrect, that's why it doesn't
> work. The argument needs to be a binary predicate, so neither of these is
> correct:
>
> // Neither of below work:
> // auto active_buttons = buttons |
> ranges::views::chunk_by(&Button::enabled, std::equal_to<bool>{});
> // auto active_buttons = buttons |
> ranges::views::chunk_by(&Button::enabled);
>
> This works:
>
> auto active_buttons = buttons | std::views::chunk_by([](auto& l, auto&
> r){ return l.enabled == r.enabled; });
>
> The problem here is not related to lack of support for projections, it's
> that you need a binary predicate and &Button:enabled is not a binary
> predicate.
>
> True, but on the other hand how does this use case differ from std::ranges::sort
or std::ranges::partition? At the end of the day std::views::chunk_by is
doing pretty much the same as partitioning and it is often used to
partition based on some class member. My simple example was with a boolean
member, but this is rarely the case - for boolean members
std::ragnes::partition is sufficient. In the real code I struggled with in
my project I had to sort my container of structures based on some "color"
member (for which it was trivial with the projection overload) and then I
used chunk_by on the same member to execute an action. If there had been
only two colors, I could have used a single std::ranges::partition
algorithm to achieve the same. Consider something like this:
https://godbolt.org/z/KdWh3sEdv.

>
>
>
>> On Wed, 13 Sept 2023 at 20:10, Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
>> wrote:
>>
>>> On Wed, Sep 13, 2023 at 11:20 AM Grzegorz Sikorski via Std-Proposals <
>>> std-proposals_at_[hidden]> wrote:
>>>
>>>> I started to notice a pattern that appears in many places. Consider a
>>>> code like this:
>>>>
>>>> struct Button {
>>>> std::string name;
>>>> bool enabled;
>>>> };
>>>>
>>>> auto get_enabled(const auto& buttons) {
>>>> return std::ranges::views::filter([](const auto& button) { return
>>>> button.enabled; });
>>>> }
>>>>
>>>> What I find useful is to add something like
>>>> https://godbolt.org/z/7Y5bdeddd. Projection support could also be
>>>> useful in some other std::ranges::views namespace members.
>>>>
>>>
>>> Nit: Write `std::views::filter`, not `std::ranges::views::filter`.
>>> Core confusion: Are you asking for C++20 to support std::invoke'ing a
>>> member data pointer? Because it already does. Example:
>>> https://godbolt.org/z/sqjxG3dW5
>>> return std::views::filter(&Button::enabled);
>>>
>>> –Arthur
>>>
>> --
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>>
>